220 CMR, § 26.07
A security deposit required by any company pursuant to 220 CMR 26.00 shall be payable in cash, certified check, bond, irrevocable letter of credit, or any other guaranteed means of payment. Where such deposit is paid by means other than cash, such means shall be renewed as necessary in order to maintain the security deposit. A company may condition service upon the verification of the enforceability of any non-cash means of payment, provided that such non-cash means is not enforceable on its face or does not appear to be a generally accepted manner of guarantee. A company shall have available to it all forms of enforcement provided for in 220 CMR 26.00 if such non-cash security deposit is determined to be unenforceable and is not cured within ten days of a request by a company.
220 CMR, § 26.07